Weird artifacts when rendering on Nvidia GPU
Since you are reducing the z axis to 0 the top and bottom faces are z-fighting with each other.
I get the exact same thing on my end, by the way, and I'm also perplexed as to why the CPU doesn't face this issue.
I tried to work out a solution using Backfacing, but failed.
Instead of leaving the z disconnected you can divide it by some large number and get a workaround.